There's
More to Light Than Meets the Eye

Humpty-Dumpty
White Light

It is difficult to
do experiments with K­12 students which demonstrate that light involves electric
and magnetic fields. Most experiments with electromagnetic radiation study the
different wavelengths of visible light. Visible light ranges from 400 nanometers
(400 billionths of a meter, corresponding to violet light) to 700 nanometer (red
light). The classic demonstration is to make a rainbow, or spectrum, out of white
light (see activity). The Exploratorium's Science
Snackbook has other demonstrations that involve mixing colors of light.

Experiments with
ultraviolet and infrared light reinforce the idea that electromagnetic radiation
extends beyond the visible wavelength range. Experiments with ultraviolet light,
in particular, are visually impressive. For advanced students, these experiments
can show how light reveals the structure of atoms.

Fluorescence
with ultraviolet light

Light just beyond
the violet edge of the visible spectrum is called ultraviolet light. As
anyone who has been sunburned knows, ultraviolet photons carry more energy
than the visible variety. You should protect your eyes by wearing UV-absorbing
goggles. Another safe alternative is a light-viewing box, available from scientific
supply houses or easy to build yourself. In such a box, the UV light is directed
away from the eyes toward the black interior of the box, where it is absorbed
and safely emitted at longer wavelengths.

The best UV light
sources produce both long-wavelength (300-400 nanometer "black light") and short-wavelength
(less than 300 nanometer) light. Fluorescent minerals or dyes, which absorb
the UV light and emit it as visible light, create a spectacular demonstration.
If you switch the UV light from long-wavelength to short-wavelength, you will
see a difference in the color (wavelength) of the emitted light. The phenomenon
of fluorescence involves the structure of atoms (see Inside an
Atom).

Blocking UV
light

Visible light penetrates
glass. We can see it! But UV light does not. Put a fluorescent mineral inside
a light box containing a UV source. Then cover the mineral with a glass jar. Is
the rock still fluorescent? How quickly does the fluorescence turn off? Does it
make a difference if the UV light is long-wavelength or short-wavelength? Other
materials, such as a plastic cup or UV-absorbing goggles, can also be tested to
see whether they block UV light.

Infrared light

Light just beyond
the red edge of the visible spectrum is called infrared light. Its photons
carry less energy than those of visible light. Our hands are better detectors
of IR light than our eyes. Things that emit in the IR feel warm: fire, electric
heaters, Sun-baked pavement.

The ASP's Project
ASTRO activities handbook, The Universe at Your Fingertips, describes
an experiment to test whether there is light below the red edge of the visible
range. This experiment involves three thermometers, which measure the temperature
of the air where the experiment is being done. Break sunlight into a spectrum
using a prism and place the thermometers at three points in the spectrum: one
in the violet range, one in the yellow range, and one just barely beyond the
red end. What do the thermometers read?

In
the Dark

It is easy to have
misconceptions about a topic as abstract, yet misleadingly common sensical, as
light. Drawing out these misconceptions is an important first step to rebuilding
students' knowledge. Students commonly have trouble understanding that the visible
window is just one small part of a continuum of electromagnetic radiation (see
Dreams of Fields). The reasons may include:

X-rays and ultraviolet
radiation are hazardous. Visible and infrared are life-saving. So how can
they all be the same type of radiation?

We see visible
light. Therefore, it must be different from the other radiation we can't see.

Another source
of confusion is the different results students get when mixing colors of light,
as opposed to mixing colors of pigments. Mixing red, green, and blue light makes
white light because these are the wavelengths that comprise white light. By
contrast, mixing red, green, and blue paints creates black  the absence
of light and color. This happens because the red paint absorbs all colors except
red light; the green and blue pigments absorb the red. Only when we confront
our misconceptions can we can begin to replace them with facts.

DEBRA FISCHER
is a graduate student in astronomy at the University of California in Santa
Cruz. As if analyzing star formation weren't enough, Fischer volunteers for
the ASP's Project ASTRO, coordinates the science fair at Commodore Sloat Elementary
School in San Francisco, and runs Lick Observatory's "Ask an Astronomer" program.
Oh, and she's the mother of three. Her email address is fischer@ucolick.org.
The authors would like to thank Roy Bishop of Acadia University for reviewing
an earlier draft of this article.

Inside
an Atom

Light lets us peek
inside the atom -- if we know how to look. According to the simple Bohr model,
an atom consists of a nucleus around which electrons buzz in orbits. Each electron
orbit represents a discrete energy level; the lowest energy levels are those
closest to the nucleus. It takes energy to move up to a higher level.

A photon of light
provides the energy that an electron needs to climb up a level. If the photon
comes close enough to an atom, it can be absorbed by the atom, pushing the electron
up (see diagram below). Depending on how much energy the photon contains, the
electron might move up one, two, a few, or many energy levels. If the energy
is great enough, the electron may go flying out of the atom altogether.

Atoms are good
at absorbing energy, but not so good at holding on to it. Within a few billionths
of a second, the electron comes bumping back down to a lower level. Each bump
is a step from a higher energy orbit to a lower energy one. At each step, the
atom must spit out a photon whose energy equals the energy difference between
the two levels.

The key thing is
that the atom does not have to release a single photon of light. It can, and
often does, release light in a whole series of steps. In this case, the total
energy from all the steps must equal the energy of the initially absorbed light.
Because there are several outgoing photons, each individual photon is lower
in energy -- therefore, longer in wavelength -- than the incoming photon. That's
how atoms can turn ultraviolet light into visible light.

Emission
from an atom. The Bohr model of the atom gives a rough idea of what
happens when an atom absorbs or emits light. The atom looks like a miniature
solar system: a nucleus surrounded by electrons in various orbits. In
the top case, a photon of short-wavelength light is absorbed by the
atom, causing one of its electrons to jump farther away from the nucleus.
If this electron falls back down to its original position, it emits
a photon of the exact same wavelength. In the bottom case, the electron
does not fall to its original position, but rather to an intermediate
position. In this case, it emits a photon of lesser energy (longer wavelength).
Diagram by Debra A. Fischer.

Dreams
of Fields

Opposites
attract; like repels. What would pop songs and love sonnets do without the
metaphors of magnets? Most of us have played with fridge magnets or compasses;
we have seen magnetic poles with the same polarity repel each other and magnetic
poles with opposite polarity attract each other. It all depends on the invisible
magnetic fields.

Although the concept of a field is abstract, it is easy to envision when you
try to push two like magnetic poles together. The magnetic fields penetrate
space. They contain energy -- the ability to do work. Slide two magnets with
the same polarity toward each other on the surface of a table until they are
uncomfortably intimate. If you let go of the magnets, they will scoot away
from each other. The energy in the magnetic field is doing work on the magnets.

An
analogous situation exists for electric charges. Similar charges repel; opposite
charges attract. As with magnetic poles, electric charges are accompanied
by electric fields that penetrate space. A negatively charged electron is
pulled by the electric field of a positive charge and repelled by the field
of a negative charge. Electric fields become even more interesting when they
penetrate materials, such as metal wires. There they exert a force that causes
the electrons to move through the wire -- the phenomenon of electricity.

But that's not all. Perhaps the most amazing property of electric and magnetic
fields is the way they interact with each other. If you take a magnet and
plunge it through a loop of wire, an electric field is created. We know an
electric field is created because it forces the electrons in the wire to move;
we can measure the resulting current. In fact, this is the principle used
by electric generators in power stations.

Likewise,
moving charges create a magnetic field. To observe this, build a simple circuit
with a piece of wire and a battery. Connect one end of the wire to the positive
terminal of a 9-volt battery and the other to the negative terminal. Place
a compass next to the loop of wire and watch the compass needle move as you
connect and disconnect the wire from one of the battery terminals. An important
ingredient in both of these experiments is the variation of the fields. Static,
unchanging magnetic fields don't spawn electric fields, and steady electric
fields don't create magnetic fields.

Once
created by a moving magnet or changing electric current, a field can break
free of its source. It departs and sails through space like a thought without
a thinker. And that is what we call light. Light and other forms of
electromagnetic radiation contain both electric and magnetic fields that oscillate
in strength. A change in the electric field creates a magnetic field. In return,
the oscillating magnetic field creates an electric field. The two fields become
entwined in a cyclical dance, each one pushing and then being pulled by the
other.

Incredibly,
no energy is lost in this process. In a vacuum, an electromagnetic wave would
travel forever without losing any energy. It disappears only when it is absorbed
by matter -- for instance, a hand that intercepts sunlight and becomes warm.
This process of transporting energy is called radiation.